Conspiracy theories point accusing fingers at authorities, and offer alternatives to official explanations. Scholars have argued that in doing so, they may therefore subvert social systems and undermine confidence in established political, health and environmental positions. In this thesis we empirically put these arguments to the test. In four experiments, we found that exposure to conspiracy theories reduced people’s intention to engage in (a) the political system, (b) environmentally-friendly initiatives and (c) childhood vaccination (Chapters 2 and 3). Ironically however, instead of undermining the social status quo, we found in four experiments that conspiracy theories appear to bolster satisfaction with social systems. They appear to do so because they explain tragedies, disasters and social problems on the actions of destructive individuals and groups, rather than inherent flaws in society. By drawing attention away from the deeper limitations of social systems, conspiracy theories may therefore reduce, rather than increase, the likelihood of social and political change (Chapter 4). Finally, we found that once people have been exposed to conspiracy theories, the negative effects are difficult to attenuate. In two experiments we tested interventions based on counter-arguments (e.g., that vaccines are safe instead of harmful) and a pre-warning that detailed people’s tendency to rely on retracted information. However, both were found to be ineffective in improving intentions to vaccinate a fictional child (Chapter 5). Overall, the research outlined in this thesis highlights some of the potentially damaging consequences of conspiracy theories. This research opens up new avenues for enquiry and calls for ongoing investigations to address the growth of conspiracism in society.